1. Field of the Invention
The present invention relates to a magnetic recording medium applied to a hard disc drive and a method of manufacturing the same medium.
In recent years, reduction in size and enlargement of capacity are essential requirements for a hard disc drive which plays the concentrated role of an information storage apparatus. These requirements can be realized by enhancing the recording density of a magnetic disc medium. In order to increase the recording density, it is requested to realize reduction in thickness of a magnetic film, improvement in resolution, high coercive force and low noise.
2. Description of the Related Art
As the recording density of a magnetic disc medium becomes high, an area of one bit on the medium is reduced. In such background, it is required, for acquisition of an output, to make the magnetic layer thinner corresponding to a reduction in the size of a bit. Thereby, a semi-circular magnetic field can be assured and thereby leakage magnetic field generated from the magnetization area of one bit can be obtained. Moreover, since it is also required to narrow the bit interval, improvement in the magnetic domain structure in the magnetization transition area, scale-down of crystal grain corresponding to reduction in thickness and noise reduction by reduction in magnetic mutual operations among the particles or the like are necessary.
As a magnetic layer of a medium, a CoCr group alloy polycrystalline film formed of three or more elements has been used as a material of the magnetic layer. In the existing medium, Cr included in the magnetic grains of a magnetic layer is segregated into the crystal grain boundary and this grain boundary is non-magnetized to reduce mutual operation among the particles. In the related art, in order to promote this segregation, adding coefficient of Cr to alloy is increased, Ta or the like is added and a substrate is also heated during the film forming process.
As explained above, while a means for segregating Cr included in the magnetic grains into the crystal grain boundary and non-magnetizing this grain boundary region is introduced, it is essential to use the CoCr group alloy magnetic material as a material of the magnetic layer. However, when reduction in thickness of magnetic layer and scale-down of crystal grains are promoted, a volume of individual magnetic grains is reduced, thermal disturbance is finally generated to result in the super-normal magnetization and the magnetic recording condition can no longer be maintained. In order to reduce the volume of individual magnetic grains as much as possible to a small amount, it is best to use a magnetic material having a higher anisotropic constant Ku. In the Co-based magnetic material, a value of Ku is lowered by as much as about one digit by adding Cr to the discrete element of Co. Therefore, use of the CoCr group alloy will make it very difficult to maintain the value of Ku. Moreover, the discrete element of Co will bring about a problem that corrosion proof characteristic is deteriorated.
An object of the present invention is to provide a magnetic recording medium having higher recording density. Moreover, another object of the present invention is to realize a reduction in thickness of a magnetic film of a magnetic recording medium. Moreover, the other object of the present invention is to reduce noise of the magnetic recording medium itself.
In the present invention, unlike the CoCr group alloy used as a material of a magnetic layer, a Co-based alloy, where a non-magnetic element different from Cr is added to the Co discrete element, is used as a material of the magnetic layer. According to the present invention, reduction of the value of Ku due to the formation of alloy is controlled, and thereby a problem in reduction of thickness of magnetic layer in a magnetic recording medium can be solved.
However, when a Co-based alloy not including Cr is used as a material of the magnetic layer, Cr included in the magnetic grains is segregated into the crystal grain boundary and the grain boundary region can no longer be non-magnetized. Therefore, in the present invention, a Cr-based non-magnetic material is used as an underlayer. According to the present invention, the grain boundary region can be non-magnetized by segregating Cr to the crystal grain boundary from the underlayer. In practice, diffusion of Cr into the crystal grain boundary of the magnetic layer from the underlayer is induced with post-annealing. As a result, the magnetic layer has a structure in which Cr exists only at the area near the crystal grain boundary, and thereby non-magnetization of the grain boundary region of the magnetic layer can be realized. Since the grain boundary region of the magnetic layer is non-magnetized, magnetic mutual operation among the grains can be reduced, and noise of magnetic recording medium can also be reduced.